The infra-red spectra of layer silicates

The infra-red absorption bands arising from the structural hydroxyl groups of a number of dioctahedral and trioctahedral layer silicates are reported with grating resolution in the 3750–3500 cm⁻¹ region. The frequency differences and orientation behaviour of these bands are discussed in terms of the known structures of the minerals. The effect of the deviation from hexagonal symmetry shown by the tetrahedral layers of dioctahedral minerals on their vibrations is theoretically analyzed, and shown to account for some of the principal differences between the spectra of dioctahedral and trioctahedral layer silicates in the 1300 to 400 cm⁻¹ region. Comparison of the spectra of random and oriented specimens of dioctahedral layer silicates in this region permits an assignment of some of the principal absorption bands to particular vibrational modes of the crystal lattice, and reveals some surprising differences between corresponding vibrational frequencies in related structures. Effects on the spectra of Al-for-Si substitution in the lattice of both dioctahedral and trioctahedral minerals are reported. A striking effect of particle size on the appearance and position of some of the stronger absorption bands of the kaolin minerals is related to the direction of the dipole moment change associated with these vibrations. Minerals examined include pyrophyllite, beidellite, rectorite, muscovite, margarite, montmorillonite, nontronite, celadonite, lepidolite and kaolins in the dioctahedral series, and talc, hectorite, saponite, phlogopite and biotite in the trioctahedral series.